This invention relates, in general, to integrated circuit (IC) technology and, in particular, to the method of fabricating packages for IC devices (chips).
More specifically, the invention relates to a method and means for maintaining alignment of components of an IC package when subjected to high temperature during manufacture, such as, when bonding a heat sink to a substrate, or when encapsulating a chip (die) attached to a lead frame with plastic in the manufacture of IC packages.
To explain the invention in the manufacture of IC packages which have heat sinks and substrates, it is conventional to attach a chip, either by wire bonding to conductor leads disposed on a flat substrate of semiconductor material, or to connect the chip directly to such leads on the substrate. The leads conventionally terminate as pads at the edges of the substrate, and in the case of a pin type package, pins are connected to these pads and, in the case of a leadless package, these pads are the terminals themselves. In the latter case, the position of the pads corresponds to the position of the conductors in a mounting socket in which the package is to be mounted for use. A typical example of a leadless package having such terminal pads and a mounting socket is shown in the U.S. Pat. No. 3,946,276 to Braun et al
This patent also shows a heat sink, of copper alloy, or aluminum alloy, attached to the substrate for dissipating heat generated during operation of the device. This heat sink is suitably bonded to the substrate as by soldering and, to do this, the temperature of the partially assembled package is raised above the melting point of the solder. However, since the material of the heat sink has a thermal coefficient of expansion which is substantially different from the substrate material, an excursion in the location of the terminal pads may occur especially if the two items were fixtured about their edges. If this does occur, a good electrical connection of all the terminal pads with their respective conductors in the socket becomes virtually impossible.
Accordingly, it becomes apparent that there is a need to provide a means by which a substrate and a heat sink can be bonded together, allow for the difference in the coefficient of expansion, yet maintain the two pieces aligned during the terminal expansion and contraction that occurs during bonding and other steps in the fabrication of an IC package so that there is minimal displacement of the terminal pads from their desired location in the final package.
Now in the manufacture of plastic encapsulated IC packages, the chip (die) is attached to the conductors of a lead frame either by wires between the die and the conductors or directly between the die and the conductors, or via intermediary lead frame devices, which are known as beam leads. The conductors are arranged for connecting the package to a socket or to a similar conductive pattern on a substrate such as a PC board similar to that described in connection with the heat sink package. The lead frame with the die attached is placed in a transfer mold which has a mold cavity which surrounds the die and a part of the leads in the process of forming the package. Such a package may or may not have a heat sink attached thereto, and with or without a heat sink, the problem of the excursion of the conductors of the lead frame during high temperature encapsulation in a mold needs to be solved.
It should also be pointed out that another problem with the movement of the terminal pads and conductors of a lead frame during the fabrication of an IC package whether a heat sink type or not, lies in the fact that often the location hole (or the datum hole, as it is sometimes called), which receives a pin in the mounting socket to thus locate the package in the mounting socket, is not necessarily in the geometric center of either the mounting socket or the package. Since a relatively flat material such as the substrate and a heat sink, when heated and cooled, expands and contracts all directions from its center, that the fact that the datum hole (point of reference) is not in the center of the heat sink or substrate complicates the matter of compensating for the possible movement of the pads or conductors during the thermal expansion and contraction occurring during heating and cooling of the package.
Therefore, it is a first object of the present invention to provide a method and means for maintaining alignment of the various parts of an IC package during the heating and cooling steps in the process of fabrication; such method and means to have universal application whether the package of the heat sink type, pin type or leadless, and whether the package is plastic encapsulated with or without a heat sink.
Thus, still another object of this invention is to provide method and means by which the geometric center of parts of an IC package can be located for purposes of fabrication during heating and cooling steps notwithstanding the fact that the datum hole or locating criteria may not be in the geometric center of the parts.